This invention generally relates to methods for providing a thermally stable finish. More specifically, this invention relates to a method for providing a thermally stable finish for a plastic container.
Recently, manufacturers of polyethylene terephthalate (PET) containers have begun to supply plastic containers for commodities that were previously packaged in glass containers. The manufacturers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable, and manufacturable in large quantities. Manufacturers currently supply PET containers for various liquid commodities, such as juices. They also desire to supply PET containers for solid commodities, such as pickles. Many solid commodities, however, require pasteurization or retort, which presents an enormous challenge for manufactures of PET containers.
Pasteurization and retort are both methods for sterilizing the contents of a container after it has been filled and capped with a closure. Both processes include the heating of the contents of the container to a specified temperature, usually above 70xc2x0 C., for a duration of a specified length. In low temperature pasteurization, the bottle is generally exposed to temperatures up to 75xc2x0 C. In high temperature pasteurization, the bottle is generally exposed to temperatures greater than 75xc2x0 C. Retort differs from pasteurization in that retort applies external pressure to the container. This overpressure is necessary because a hot water bath is often used and the overpressure keeps the water in liquid form above its atmospheric boiling point temperature. During the pasteurization or retort process, the finish section of a typical PET container shrinks considerably. This shrinkage of the finish section prevents proper engagement of the finish section with the closure, which may lead to leakage or spoilage of the commodity within the PET container. It has been found that the finish section can be stabilized by inducing spherulitic crystallization into the finish.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to resist shrinkage is related to the percentage of the PET container that is in crystalline form, also known as the xe2x80x9ccrystallinityxe2x80x9d of the PET container. Crystallinity is characterized as a volume fraction by the equation:       %    ⁢          xe2x80x83        ⁢    Crystallinity    =                    ρ        -                  ρ          a                                      ρ          c                -                  ρ          a                      xc3x97    100  
where p is the density of the PET material; xcfx81a is the density of pure amorphous PET material (1.333 g/cc); and xcfx81c is the density of pure crystalline material (1.455 g/cc). The crystallinity of a PET container can be increased by mechanical processing and by thermal processing.
Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching a PET container along a longitudinal axis and expanding the PET container along a transverse axis. The combination promotes biaxial orientation. Manufacturers of PET bottles currently use mechanical processing to produce PET bottles having roughly 20% crystallinity (average sidewall crystallinity).
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. Used on amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque (and generally undesirable on the sidewall of the container). Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a heated blow mold, at a temperature of 120-130xc2x0 C., and holding the blown container against the mold for about three seconds. Manufacturers of PET juice bottles, which must be hot filled at about 85xc2x0 C., currently use heat setting to produce PET juice bottles having a range of up to 25-30% crystallinity in their sidewalls and over 30% in their finish sections. Although these hot fill PET containers exhibit a significant improvement over non-hot fill PET containers, they cannot adequately prevent shrinkage of the finish section to properly engage the closure. Depending on the diameter of the finish section, and the temperature and duration of thermal processing, these crystallized finishes have been measured to exhibit shrinkages in the range of 0.4% to 0.8%. With shrinkages in this range, an induction seal is required in addition to the closure cap to ensure the integrity of the seal. This adds cost and complexity to the processing of the container.
Thus, the manufacturers of PET containers desire an efficient and inexpensive method of providing a PET container for a thermally-processed commodity product retained within a PET container having a finish section properly sized and engaged with a closure. It is therefore an object of this invention to provide such a method.
The present invention includes a method for providing a PET container, which after thermal processing the commodity product retained within the plastic container, has a finish section defining a finish diameter of a predetermined value so as to ensure proper sealing of the container. The invention also includes a method for providing a thermally-processed commodity that overcomes the problems and disadvantages of the conventional techniques in the art.
The present invention in a preferred embodiment is a method for providing a thermally stable finish including providing a finish section, reforming the finish section, which in part requires crystallizing at least a portion of the finish such that the finish has a finish dimension that shrinks to an intermediate finish dimension. Then annealing the crystallized finish such that the intermediate finish dimension conforms to a predetermined range.
Further features and advantages of this invention will become apparent from the following discussion and accompanying drawings.